Electric discharge device



Feb. 11, 1936. v, J FRANCIS Er AL 2,030,437

ELECTRIC DISCHARGE DEVICE Filed Jan. 7, 1935 INVENTORS m H dm BY M41 ATTORNEY Patented Feb. 11, 1936 ELECTRIC DISCHARGE DEVICE Victor James Francis and John Walter Ryde,

Middlesex, England, assignors to General Electric Company, a corporation of New York Application January '7, 1935, Serial No. 750 In Great Britain January 12, 1934 4 Claims. (0]. 176-122) This invention relates to electric discharge devices of the type comprising an inner tube adaptedto carry a high pressure metal vapour discharge and a hermetically sealed envelope enclosing the inner tube similar to that disclosed in the United States Patent #1,948,261, granted February 20, 1934;, to Victor James Francis and John Walter Ryde.

In devices of the type specified part at least of the purpose of the outer envelope is to prevent and/or to equalize heat losses from the inner tube; the envelope has therefore hitherto been evacuated. Further although it is not always necessary to this function, the evacuation is usually carried so far that the pressure in the sealed envelope is much less than 1 mm. We have observed that in such high pressure metal vapour lamps a dark deposit develops during life on the inner surface of the inner tube. When the outer surface of this inner tube is in contact with metal, for example a metal band supporting it and connected to one electrode, the deposit forms most readily in the neighbourhood of the metal. But the deposit forms, although in general less rapidly,' even in the absence of any metal in contact with the outside of the inner tube. This dark deposit reduces the intensity of the light emitted by the device and lowers the efficiency of the device.

The object of the present invention is to provide an electric discharge lamp of the high vapour pressure type in which blackening of the inner surface of the inner tube is reduced to a minimum. Another object of the invention is to provide an efficient, long lived electric discharge vapor lamp. Still further objects and advantages attaching to the device and to its use and operation will be apparent to those skilled in the art from the following particular description.

We have discovered that the presence of certain gases in appreciable amounts in the outer envelope substantially retards the formation of the dark deposit on the inner surface of the inner tube. We have demonstrated that oxygen, hydrogen, air water vapor, sulphur vapor, carbon dioxide and nitrous oxide are effective for the purpose contemplated and that the effectiveness of these gases is about in the order in which they exactly the relative efiicien y of the various gases. But if efficiency is estimated by the pressure of gas required to produce a given reduction of blackening, the'following. table gives some idea of the relative efficiencies. (A gas is, of course, the more efiicient the less the pressure required to produce a given effect.)

Nitrous oxide These pressures were determined with lamps of the high vapor pressure type now known in commerce and which operated on an A. C. supply of 230 volts. It is possible that different values might be obtained with different types of lamp.

Oxygen and hydrogen are, then, the most eflicient gases. Oxygen is slightly preferable to hydrogen, because the high thermal conductivity of the latter increases the time required for the lamp to attain its steady operating state.

It is to be understood that the introduction of gas according to the invention does not abolish all sources of blackening. A part remains which is probably'due to sputtering or evaporation of the electrode and, is most marked in this neighbourhood. The gas reduces greatly (but may not abolish completely) the general blackening in regions remote from the electrode.

The cause of this general blackening is not fully understood; no theory can therefore be advanced to explain the action of the gas. But it is not unlikely that the blackening is due to some change in the inner surface of the glass in are named; oxygen and hydrogen are the most.

effective and nitrous oxide the least effective of the gases named.

It will be realized by those skilled in the artthat, since blackening is an ill-defined and not wholly regular process, it is diflicult to establish contact with the discharge, and therefore that the efliciency of various gases may vary to some extent with the nature of that glass. The glass used in the experiments summarized in Table 1 had roughly the following composition:-

The manufacture of electric discharge devices according to the invention need difier from that of known devices of the type specified only in the introduction of gas into the space between the inner tube and outer envelope. If the gas to be introduced is air, the evacuation of the envelope may be stopped when the air originally present has been reduced to the desired pressure, and the envelope then sealed off. If the gas is other than air (and permissibly even if it is air), the envelope is completely evacuated, the gas introduced to the desired pressure, and the envelope sealed off. i

In the drawing accompanying and forming part of this specification an embodiment of the invention is shown in a front elevational partly sectional view.

In the drawing, the luminous tube comprises internal envelope of glass I having approximately spherical ends} into which are sealed the leading-in wires 3. To these leads are spot-welded the thermionic electrodes each consisting of a short refractory rod 5 composed of a mixture of calcium and barium oxides surrounded by a tungsten spiral 4; two turns 6 of the spiral, free of the rod, are used as the curved part of the leads for preventing the conduction of excessive heat to the seals. The electrodes are mounted very close to the spherical ends of the tube, being not more than 10 mm. from the wall. In' order further to promote the heating of the ends of the tube, these may be covered with caps of thin metal foil.

The internal envelope I is arranged within an outer envelope I (shown as partly cut away in the,

drawing) having a foot-tube 8 and a standard form of lamp cap I! at one end. The leads 9 for the discharge tube are brought through the foottube in the ordinary way, and connected to the leads 3 sealed into the inner envelope; these leads 9 may conveniently carry wire rings l3 which bear against the inside of the outer envelope and serve as supports for the inner envelope.

The internal envelope contains enough mercury to give a pressure of about 1 atmosphere when operating on a current of from 2 to 3 amperes-in the case of a tube about 17 cm. long by 3 cm. diameter, for example, about 0.2 to 0.25 gm. of mercury are adequate-together with a few mm. of argon to facilitate starting. Furthermore, to render the tube self-striking, a wire I!) is carried from one of the leads along the outside of the inner envelope to a point near the other electrode of the tube and there wound round the outside of the inner envelope to form a conductive ring I I. Alternatively an auxiliary electrode may be put inside the tube, near one of the main electrodes and connected to another main electrode in a known manner by a high reistance. The use of the outer envelope with these forms of starting arrangement is particularly convenient, for it permits live conductors to be run along the outside of the discharge tube proper, without any danger that they may be accidentally touched.

The outer envelope 1 is exhausted after assembly and a quantity of gas, such as oxygen or hydrogen, at a pressure of approximately 10 mm. is introduced into said envelope I and said envelope 1 is then sealed off. The presence of the gas in I the outer envelope 1 retards the formation of dark deposits on the inner surface of the inner envelope I as pointed out above.

While I have shown and described and have pointed out in the annexed claims certain novel features of the invention, it will be understood that various omissions, substitutions and changes in the forms and details of the device illustrated and in its use and operation may be made by those skilled in the art without departing from' the broad spirit and scope of theinvention.

What I claim as new and desire to secure by Letters Patent of the United States is:--

1.- An electric dischargewdevice comprising a container, electrodes sealed therein, a quantity of vaporizable material therein, a sealed envelope for said container and a gas in said envelope under a pressure greater than 1 mm. to retard the formation of dark deposits on the inner surface of said container.

2. An electric discharge device comprising a container, electrodes sealed therein, a quantity of vaporizable material therein a sealed envelope for said. container and oxygen in said envelope at a pressure greater than 1 mm. to retard the formation of dark deposits on the inner surface of said container.

3. An electric discharge device comprising a container, electrodes sealed therein, a quantity of vaporizable material therein, a sealed envelope for said container and hydrogen in said envelope at a pressure greater than 1 mm. to retard the formation of dark deposits on the inner surface of said container.

4. An electric discharge device comprising a container, electrodes sealed therein, a quantity of vaporizable material therein, a sealed envelope for said container and air in said envelope at a pressure greater than 5 mm. to retard the formation of dark deposits on the inner surface of said container.

VICTOR JAMES FRANCIS. JOHN WALTER RYDE. 

